1. Field of the Invention
The present invention relates to an imaging apparatus having photoelectric conversion elements and a method thereof and image sensing system, and more particularly to high-quality electronic zoom utilizing a CMOS image sensor.
2. Description of the Related Art
Conventionally, CCD image sensors and CMOS image sensors are widely used as solid-state image sensing elements. In a CCD image sensor, a photoelectric conversion element arranged in each pixel of the image sensing plane converts light into signal charges. The signal charges are read out from all pixels to the CCD simultaneously and transferred. The CCD image sensor converts the transferred signal charges into an electrical signal and outputs it. In a CMOS image sensor, a photoelectric conversion element arranged in each pixel of the image sensing plane converts light into signal charges. The signal charges are amplified in each pixel and output. As characteristic features, the CMOS image sensor can selectively read out (to be referred to as “partial readout” hereinafter) image signals of photoelectric conversion elements in an arbitrary pixel region, unlike the CCD image sensor that simultaneously reads out image signals of photoelectric conversion elements for an entire pixel region (Japanese Patent Laid-Open No. 2005-94142).
FIGS. 1A and 1B are overall diagrams showing an electronic zoom operation utilizing the features of a CMOS image sensor. FIG. 1A shows readout in a normal mode. FIG. 1B shows readout in a 2× zoom mode. Referring to FIGS. 1A and 1B, an imaging unit 101 has a plurality of pixels arrayed in a matrix. FIGS. 1A and 1B illustrate an example of an imaging unit having 10×10 pixels. However, the imaging unit need not always include 10×10 pixels and can have an arbitrary number of pixels.
In the normal mode, the CMOS image sensor executes readout at a thinning ratio of 2; that is, it reads out 4×4 alternate pixels in a range indicated by a bold frame 102 including 8×8 pixels, as shown in FIG. 1A. In the zoom mode, the CMOS image sensor reads out 4×4 pixels continuously arranged at a central portion 103 in the bold frame 102 including 8×8 pixels. In this case, it is possible to display an enlarged image of the central portion 103 by readout at a thinning ratio of 1. Since the number of pixels read out in the zoom mode is equal to that in the normal mode, it is unnecessary to increase the number of pixels by signal processing. This enables high-quality electronic zoom.
However, in storing signal charges in the CMOS image sensor, the signal charge storage period shifts between the lines of the image sensing plane. This storage time shift between the lines causes a shift of the readout period of one line. The readout period of one line—that is, the period from the start of image signal readout of one line to the start of image signal readout of another line—is given byReadout period of one line=HBLK×α+Skip×β+the number of horizontal pixels×reference clock time  (1)where α and β are determined by the addition method in the vertical direction. HBLK is the horizontal blanking period. Skip is a period (skip period) necessary for skipping one line by thinning. (The number of horizontal pixels×reference clock time) corresponds to a vertical pixel readout period shown in FIGS. 9 and 11. As indicated by equation (1), the readout period of one line is represented by the sum of three periods: horizontal blanking period, skip period, and horizontal pixel readout period. Addition average of two vertical pixels will be explained as an example with reference to FIG. 2.
In the example shown in FIG. 2, α is 2, and β is 1. The time necessary for transferring horizontal pixels depends on the reference clock time. The readout period of one line can change depending on the addition method in the vertical direction or due to changes in a driving mode such as a driving frequency. The storage start times at the top and bottom of the image sensing plane can vary due to changes in a driving mode. FIG. 3 is a diagram showing a rolling shutter operation when changing the driving mode. FIG. 3 shows an example when the readout period of one line is longer than that before changing the driving mode. In the example shown in FIG. 3, the storage time difference between the top and bottom of the screen is larger than that before changing the driving mode.
FIG. 4 shows an example when the driving mode changes in EVF (Electronic ViewFinder) display or a moving image process such as moving image recording. Note that the driving mode changes at a timing during the VBLK (vertical blanking) period after readout finishes.
In FIG. 4, the abscissa represents the time, and the ordinate represents the readout position in the CMOS image sensor. Frames 505 to 511 represent storage timings and storage times at line positions in each driving. FIG. 4 shows the storage times of the frames 505 to 511 when the CMOS image sensor is driven in a driving mode A up to a time t1 (501) and then driven in a driving mode B from the time t1 (501). Reference numeral 502 indicates a vertical synchronous signal VD. In the driving mode A, the sensor executes full screen readout 503. In the driving mode B, the sensor executes partial readout 504. In the frame 505 in the driving mode A, a time from a reset timing 514 to a readout timing 515 corresponds to a storage time 516. Similarly, in the frame 509 in the driving mode B, a time from a reset timing 517 to a readout timing 518 corresponds to a storage time 519. This also applies to the frames 506 and 507 in driving mode A and the frames 510 and 511 in the driving mode B.
Since the reset start time in the frame 508 is earlier than the time t1, the reset timing in the period before the time t1 is the same as in the driving mode A. From the time t1, the mode changes to the driving mode B. For this reason, the readout timing is different from that in the driving mode A. As a result, in the frame 508 upon drive switching, the storage time difference between the top and bottom of the screen changes from the switching time t1.